1. Field of the Invention
This invention relates to methods for refining oils and fats.
2. Description of the Prior Art
Alkali refining methods have been used widely in refining oils and fats. When this method is applied to the refining of oils containing large amounts of gummy substances, the oil is usually subjected to a pretreatment such as degumming. The degumming is usually carried out by adding water, acid or other chemicals to the crude oil and then separating the gummy substance with centrifugation in the hydrated or coagulated form.
However, this degumming process is usually not sufficient to entirely remove the gummy substance from the oil and traces of the gummy substance remain in the oil. Therefore, if required, the partially degummed oil is further treated with acids such as phosphoric acid, and then deacidification is carried out by contacting the oil with an aqueous solution of alkaline compound in order to neutralize free fatty acid and other acids as well as to saponify, hydrate and coagulate the gummy substance left in the oil. The influence of gum on the quality of the oil is eliminated entirely by these treatments as the gummy substances are removed from the oil.
Although the alkali refining method is very advantageous in removing the gummy substance from the oil, this method has the following disadvantage. The free fatty acid in the oil reacts with the alkali to form soap and is separated from the oil phase in the deacidification process of the alkali refining method. The separated soap called "soapstock" is usually decomposed by an acid such as sulfuric acid in order to recover the fatty acid. In the course of this acid treatment, a large amount of waste water called "acid water" is produced which is high in acidity and BOD value. The deacidified oil after separation of soap from the oil is then washed with water. Therefore, a large amount of waste water is produced. As it contains a large amount of oil, it is not allowed to discharge the waste water from the factory without treatment by the pressure floatation method or by the activated sludge process. Therefore, the alkali refining process is not always advantageous, since a large investment is required in the refineries to avoid environmental pollution according to the various regulations which are becoming stricter.
The alkali refining process has further disadvantages which are the loss of neutral oil entrained in the soap and the loss of neutral oil saponified in the course of deacidification accompanied by the neutralization of the free fatty acid.
In the steam refining method, crude or degummed oils and fats are directly decolorized and deodorized (as well as deacidified). Since this method does not have the disadvantages of the alkali refining method such as the loss of neutral oil and the polution by waste water, this process has many advantages over alkali refining process. However, the steam refining method does not include sufficient degumming process as in the alkali refining process, therefore it is necessary to treat the oil with a complete degumming process before steam refining, to remove the gummy substance exhaustively from the oil.
The complete degumming before steam refining is practically difficult and it is not always done sufficiently despite the use of various degumming agents such as acids and salts. Even when the oil seems to be sufficiently degummed, decolored and deodorized judging from the appearance, the oil is often far inferior to the oil refined by the other method in oil flavor, especially when the oil is heated. This shows that the steam refining method still has many problems to be solved.
It is also found that the oil refined by steam refining method is often inferior in flavor and odor when heated as compared to the oil refined by the alkali refining method, even when the oil to be treated does not contain much gummy substance originally, such as, palm oil and lard. From this point of view, it is presumed that the use of an alkaline solution in refining oil plays a part not only in removing the gum from the oil but also in eliminating or inactivating the factors affecting the flavor of the refined oil. Although the steam refining method has many advantages over alkali refining from the economical point of view, the former has still many problems to be solved. Oil refineries are obliged, at present, to utilize the conventional alkali refining method, even though this method has the above-noted disadvantages.
A need therefore, continues to exist for an oil or fat refining process which avoids the production of waste water, lowers environmental pollution, diminishes the loss of neutral oil in the refining process and is economical in operation.